No 2-3s is not normal. With preloading on, the time-to-next-image is essentially zero. Your system specs look good, not sure what's going on on this computer.

For 5MB-ish images (I assume 10-12MP) I recommend that you set the preload slider to about 1/3 of the range, with preloading on, and since your disks seems fast you can probably enable the parallel loading option.

In fact all these options are best left to their default. They exist to accommodate very atypical cases such as huge images (think 100MP+) or ultrafast scrolling of small images (think 30 per second, when disk bandwidth permits).